This invention concerns an air balancing hoist in which regulated air pressure is applied to a movable element such as a diaphragm or piston to exert an axial force on a drum reel, which is in turn fixed to a ball screw or nut which results in a wind-up torque countering the wind out torque created by a load supported on a cable wound on the drum reel. The regulated air pressure is typically set at a level to counter all but a small proportion of the weight of the load, so that the load may be easily handled normally.
Such hoists are shown in U.S. Pat. Nos. 3,260,508, 3,428,298; and 3,526,388.
A safety hazard is inherent in these designs in that if the load is suddenly lost, as by the cable snapping, the regulated air pressure will cause the drum reel to rapidly wind up, with the end of the cable being swung wildly about.
Attempts have been made to prevent this from happening by automatically reducing the applied air pressure in this event, but rapid rotation of the drum reel has usually been already started such that its momentum continues the too fast cable wind up.
Safety devices have heretofore been devised for various reel and hoist devices, as for example the devices shown in U.S. Pat. Nos. 3,311,351; 3,160,360; 4,566,375; 4,625,933; and 4,722,422. However, none has been devised for regulated air pressure, ball screw operated balancing hoists of the type described.
Accordingly, the object of the present invention is to provide a simple yet effective safety device for preventing sudden over speeding of an air balancing hoist of the type using a ball screw arrangement for driving and holding a drum reel.